This invention relates to a fire retardant impregnate for lumber and plywood and other wood products which does not degrade the wood substrate when it is subjected to prolonged high temperatures and humidity.
Wood materials used in the construction of, for example, roofs are typically subjected to high temperatures and may also be subjected to high humidity levels. While it is desirable to use pressure treated lumber and plywood and also to treat the same with fire retardant compositions, prior art treated lumber and plywood suffered degradation due to thermal exposure during use. In fact, thermal degradation in roof sheathing and roof truss lumber has been observed within as few as two to five years after installation.
It has been speculated in an article appearing in xe2x80x9cThe Forest Products Journal,xe2x80x9d volume 47, No. 6, pages 79-86 (published in 1997) that the presence of phosphorus containing compounds in fire retardant compositions leads to the thermal degradation of the substrate plywood or lumber. Many prior art fire retardant compositions used in treating lumber contain phosphoric acid or reaction products of phosphoric acid such as ammonia phosphate.
It has been hypothesized that the combined affects of phosphate retention in the wood and accumulated thermal exposure result in the formation of phosphoric acid or one of its ionization compounds which, when combined with prolonged heat, degrade the treated wood construction by reacting with wood carbohydrates.
Examples of such prior art fire retardant compositions are shown in U.S. Pat. Nos. 4,373,010; 4,514,326; 4,725,382; 5,009,964 and 5,151,225. These patents all describe proposed commercial fire retardant formulations for the pressure treatment of wood products and all contain phosphoric acid or reaction products of phosphoric acid.
It has been proposed however that the problem of thermal degradation can be solved by eliminating phosphorus containing compounds from wood impregnates and also by eliminating ammonia and ammonia salts of strong acids.
It was further described in the above article that borate-based buffers could mitigate the thermal degradation as an additive to phosphate fire retardant compositions.
In U.S. Pat. No. 4,801,404 a fire retardant composition is described which includes a dispersion of boric acid and a small amount of a borate salt in an aqueous solution. The boric acid is only slightly soluble in water and an alkaline agent was used with the aqueous dispersion to react with the boric acid. Examples of such alkaline agents includes phosphate or ammonia borates.
In U.S. Pat. No. 4,935,457 a fiber board binder is provided which includes as an impregnate a boric acid derivative together with an alcohol and a water soluble amine containing compound. In another example in U.S. Pat. No. 4,961,865, sodium borate is used in combination with sodium chloride, magnesium chloride, sodium sulfate and water as an impregnate for wood and other cellulosic materials.
Further, in U.S. Pat. No. 5,404,555 there is described an impregnate for celluosic materials which includes ammonia sulfate, boric acid, borax, and hydrogen peroxide.
U.S. Pat. No. 4,197,235 discloses a fire retardant polyamide dry composition. In examples 1 through 3 of the ""235 patent, boric acid was dissolved in water and poured into a hot solution of melamine in water. The mixture was allowed to cool and the salt filtered off and dried. Then in Example 4, the dry salt composition was mixed with at least a polymer and extruded to form the electrical components. As such, this dry fire retardant material which is a water insoluble product is designed to be incorporated into polymeric materials, not penetrated into wood products.
U.S. Pat. No. 3,897,372 discloses a fire retardant hydrophilic foam composition for polyurethane materials. The foam, as set forth in Examples 3 and 5, comprises polyethylene glycol 1000, trimethylolpropane, 2,4 and 2,6-tolyldiisocyanate, silicone surfactant L-520, water, boric acid, melamine, and, just for example 5, alumina hydrate. The foam of the ""372 patent xe2x80x9cmay be formulated so as to be flexible, semi-rigid, or rigid in nature and to be of primarily open cell or primarily closed cell structure as desired.xe2x80x9d (Col. 6, lines 25-29) Obviously, this composition is not able to penetrate into a wood product.
None of these prior art references teaches or describes an aqueous, fluid composition for impregnating plywood, cellulosic composite products, or lumber (collectively referred to as xe2x80x9clumberxe2x80x9d) to provide fire retardant characteristics without thermal degradation which consists of a composition which does not contain compounds of phosphates or ammonia or ammonia salts of dry acids.
It has been discovered, however, that by eliminating phosphorous containing compounds and ammonia salts of strong acids from an impregnate solution, fire retardant capabilities can be imparted to the structure without the thermal degradation associated with prior art compositions and, in fact, in some tests enhanced strength was exhibited as compared with untreated wood constructions, after prolonged thermal exposure.
The composition of this invention includes nitrogen containing organic compounds and boron containing compounds with the ratio of nitrogen to boron of 1.25:1.00 to 1.75:1.00. Examples of nitrogen containing compounds are dicyandiamide, guanidine, cyanamide, urea, guanylurea, melamine, biuret and mixtures thereof.
Examples of boron compounds are boric acid, metaboric acid, tetraboric acid, boric oxide, and alkaline borates such as sodium octaborate, sodium tetraborate, sodium pentaborate and their hydrates, as well as other metallic salts of boron and oxy acids of boron. A preferred hydrate is sodium octaborate tetrahydrate, a compound commercially available under the trade name xe2x80x9cPolyborxe2x80x9d.
In one preferred embodiment of this invention the nitrogen containing compounds are a mixture of dicyandiamide and urea. Successful tests have been conducted with the composition of this invention wherein the ingredients boric acid, borax, dicyandiamide and urea are present in from about four to about twenty five sixteen percent in an aqueous solution.
Accordingly it is an object of this invention to provide a fire retardant composition for use with lumber, plywood and other wood products which will mitigate thermal degradation when the wood substrate is subject to thermal exposure.
It is another object of this invention to provide a fire retardant treatment for lumber, plywood and other wood products which is phosphate free and free of inorganic ammonia salts of strong acids.
It is another object of this invention to provide a boric acid composition, which is in an aqueous solution, free of phosphates and ammonia salts of strong acids, for use as a fire retardant treatment for lumber, plywood and other wood products, which will not cause thermal degradation of the wood substrate during prolonged thermal exposure.
These and other objects will become readily apparent with reference to the drawings and following description wherein: